The present invention relates to an encoding apparatus for making accurate measurements of a movement of a measuring object within a copying machine or precision measuring equipment.
A known encoding apparatus which can make measurements of a linear movement of a measuring object with the currently highest resolution is shown in FIG. 32. In the encoding apparatus in FIG. 32, the light emitted from a helium-neon laser light source 61 passes through a half-mirror 62 and it enters a rotary encoder disk 63 in a direction perpendicular to the surface of the rotary encoder disk 63. The rotary encoder disk 63 includes a diffraction grating A having a number of very fine radial grating slits. The grating slits are formed in the surface of the rotary encoder disk 63 so as to radially extend from the center of the rotary encoder disk 63. The rotary encoder disk 63 has a 15-mm outside diameter, and the diffraction grating A has 30,000 grating slits and a 15.7-.mu.m grating pitch at the outside periphery thereof.
As the light emitted from the light source 61 enters the rotary encoder disk 63 in the direction perpendicular to the surface of the diffraction grating A, (-1)th-order and (+1)th-order diffracted light rays are produced by the diffraction grating A and they are directed to mirrors 64 and 65 through two optical paths. A portion of the light from the light source 61 is reflected from the half-mirror 62 and the reflected light passes through a pinhole 67 and it enters a photodetector 68.
At the same time, the (.+-.1)th-order diffracted light rays are reflected from the mirrors 64 and 65, and they enter the diffraction grating 63 through the optical paths. The reflected light rays (the (.+-.1)th-order diffracted light rays) enter the diffraction grating A at the same position. The diffracted light rays are produced by the diffraction grating A in the same manner, and they enter the half-mirror 62 in the opposite direction through the optical path that is the same as the original optical path. Thus, the light rays emitted from the light source 61 and the light rays reflected from the diffraction grating 63 interfere with each other, and the interference light ray is reflected from the half-mirror 62. The interference light ray passes through the pinhole 67, and it enters the photodetector 68. Thus, the intensity of the interference light can be detected by the photodetector 68.
As the rotary encoder disk 63 rotates in a direction indicated by the arrow in FIG. 32, the diffraction grating A is moved relative to the light source. By the rotation of the diffraction grating A, the intensity of the interference light detected by the photodetector 68 is changed in response to that movement.
More specifically, in accordance with one grating pitch of the movement of the diffraction grating A in the rotating direction, the intensity of the interference light detected by the photodetector 68 is changed as much as four periods of the interference light. When the rotary encoder disk 63 is rotated by one complete revolution (equivalent to the amount of 30,000 grating slits on the rotary encoder disk 63), the photodetector 68 outputs 120,000 sine signal pulses, that is, the sine signals with a 0.052-mrad period can be obtained from the photodetector 68. Therefore, the encoding apparatus in FIG. 32 can make measurements of the number of revolutions of a measuring object or measurements of the rotation speed of the measuring object with high resolution.
However, the known encoding apparatus can provide only measurements of a linear (or one-directional) displacement of a measuring object. It is impossible for the known encoding apparatus to make measurements of a two-dimensional displacement of a measuring object.